1. Field of the Invention
The invention relates to narrow band output excimer and molecular fluorine laser systems and particularly to a pulse compression circuit configuration for reducing effects of amplified spontaneous emission and thereby improving spectral purity in the output beam.
2. Discussion of the Related Art
In a conventional excimer laser excitation circuit having a solid state pulse generator, the circuit components are typically configured to minimize the risetime of the charge and maximize the discharge speed of final pulse compression stage capacitors, or peaking capacitors, connected to discharge electrodes in the laser chamber. In this way, it is thought that an efficient gain may be achieved.
FIG. 1 schematically shows a portion of a conventional excimer laser discharge circuit arrangement. A pair of main electrodes 32a, 32b separated by a discharge area or volume 34 filled with a portion of a gas mixture that generally fills the laser discharge chamber (not shown). A pair of preionizers 36 are also shown. An electrical pulse is delivered to the electrodes 32a, 32b, 36 from the discharge circuit of which only one of several final stage capacitors, or peaking capacitors 38, of a pulse compression circuit are shown. Each of the peaking capacitors 38 has a capacitance Cp and is connected to the main electrodes 32 through an inductance Lp. Since it is typically desired to keep the inductance Lp as small as possible in accordance with the goals of fast charge risetime and fast discharge of the peaking capacitors 38, the peaking capacitors 38 are typically located as close as possible to the high voltage main electrode 32a. 
FIG. 2 illustrates exemplary waveforms achieved using the arrangement of FIG. 1, i.e., an arrangement wherein the peaking capacitors 38 are each arranged close as practical to the high voltage main discharge electrode 32a to minimize the inductance Lp between the peaking capacitors 38 and the main electrode 32a. Three plots are shown. The first labeled plot 1 is the waveform of the electrical pulse delivered to the main electrode 32a. The second labeled plot 2 is the waveform of the light pulse emitted from the laser resonator. The third labeled plot 3 is the waveform of the amplified spontaneous emission (ASE) emanating from the discharge volume. It is noted here that the amplitudes of the traces shown at FIG. 2 are not comparable because different signals were used.
The fast risetime of the excitation circuit of FIG. 1 (see FIG. 2, plot 1) leads to a fast rise of the gain which is on the order of nanoseconds (ns) or up to several tens of ns. This fast electrical pumping determines the rise of the emitted optical pulse (see FIG. 2, plot 2) in the line narrowed laser. It is recognized in the invention that the ASE is not line-narrowed and gives rise to a broadband background within the otherwise line-narrowed emission of the laser. It is moreover recognized in the invention that the gain of the laser becomes saturated by photons generated at the leading edge of the electrical pulse, and that the ASE is typically mainly emitted at the leading edge of the pulse.
After the first half round trip, e.g., after 5-15 ns, photon generation within the discharge chamber becomes predominantly stimulated emission, rather than spontaneous emission, due to its far greater cross section and the arrival of large numbers of reflected photons, which either have been or will be subject to line-narrowing before outcoupling from the resonator, unlike ASE photons. Thus, the spectral purity of the line-narrowed laser beam may generally be limited by the intensity of broadband background ASE generated at the leading edge of the electrical pulse.
It is thus an object of the invention to reduce the proportion of the ASE within the laser pulse in order to achieve greater spectral purity in the emitted line-narrowed beam.
In accordance with the above, an illumination photolithography system for producing structures on a workpiece includes an excimer or molecular fluorine laser system for delivering an output emission from the laser system to the imaging system and/or workpiece. The laser system includes a discharge chamber filled with a laser gas mixture, multiple electrodes including a pair of main electrodes separated by a discharge volume within the discharge chamber and connected to a pulsed discharge circuit for energizing the gas mixture within the discharge volume, and a resonator including the discharge chamber for generating an output laser beam.
The pulsed discharge circuit includes a high voltage power supply, a main storage capacitor which is charged by the power supply, a pulse compression circuit and a switch for permitting the storage capacitor to discharge through the pulse compression circuit to the electrodes. The pulse compression circuit includes a series of pulse compression stages each having a stage capacitance and being separated by a stage inductance.
A final stage capacitance is provided by a set of peaking capacitors connected through a first inductance to the electrodes and a set of sustaining capacitors connected to the electrodes through a second inductance substantially greater than the first inductance. The current pulses through the discharge are temporally extended relative to current pulses of a system having its final stage capacitance provided only by a set of peaking capacitors connected to the electrodes via a lower inductance. An amplified spontaneous emission (ASE) level in the laser output pulses is thereby reduced and the spectral purity is enhanced.
The additional inductance is preferably sufficient to reduce the broadband background ASE to less than 0.5%. The additional inductance is preferably more than 50% of the original inductance. The sustaining capacitors also preferably provide more than half of the overall capacitance of the final stage capacitors.